Patent GB-288238-A (GB)
Title: Improvements in television systems
IPC: H04N5/257
Filed: 1928-01-18
Abstract: 288,238. Electrical Research Products, Inc., (Assignees of Gray, F.). April 6, 1927, [Convention date]. Television.-Transmitter. In a television transmitter, the person to be televized is scanned by a thin pencil of rays which passes through a system of light sensitive cells located between the scanning disc and the object and is reflected back by the object on to the cells. The light pencil proceeds from an are lamp, is collimated so as to be slightly convergent, passes through a scanning disc formed with apertures arranged in a spiral, and through a mask which screens off all but one of the apertures at a time, and is deflected by a lens so as to pass through the focus of the lens and on to the face of the objectperson. Light is reflected thence on to three photo-electric cells 10, 11, 12 of the kind described in Specification 288,539, [Class 39 (i). Electric lamps, Arc &c.], these cells being arranged at the sides and top of the front aperture of the chamber which contains the above - mentioned apparatus The width of the light pencil is such that there is a slight overlap between the elementary bands which are successively scanned. The casing which contains the transmitter comprises iron sheeting separated by wool felt from an inner lining of copper, in order that the circuits may be screened from magnetic, acoustic and electrical disturbances and from stray light. The output current of the cells 10, 11, 12 is amplified with suppression of the direct-current and low frequency components, which represent changes in the average illumination; these are restored manually by adjustment of the potentiometers 99, 99<a> at the receiving station. In an alternative arrangement each aperture in the scanning disc may be associated with a separate lamp which is mounted behind it in the disc. Receivers. Received signals may be applied through plug-and-jack connections 33, 34, either to an apparatus 91 for viewing by a single person or to an apparatus 56 for viewing by a body of persons. The former apparatus is controlled through a valve amplifier which can be adjusted at 99, 99<a>, to supply the direct current and very low frequency components which represent variations in the average brightness of the picture. The output of the amplifier is applied to a neon lamp 25 which may either illuminate a ground glass screen or have a frosted front surface or be formed with a cathode slightly larger than the field of view (2" x 2") traced out by the scanning disc 31 behind an aperture 88 in a fixed mask 87. To obviate a grained effect in the picture the line elements successively traced out overlap slightly. The apparatus is enclosed in a sound-proof casing. Signal currents reach the alternative receiver 56 through an amplifier A45 and an oscillator and modulator OM50. The field of view comprises a large neon lamp having electrodes-elements arranged to form a grid as described in Specification 288,239. Thus, there are fifty parallel sections 111 each containing fifty external tinfoil electrodes, and each row has an internal helical electrode extending throughout its length. The output from the valve OM50 is applied through a commutator 135 to each electrode in turn, and the resulting variations in glow, viewed directly or on a ground-glass screen 57, form a re-constituted picture. The electrodes are continuously energized at the same time by an oscillator 0160 to eliminate lag in response. Circuits and screening of cells and amplifiers. Two of the photo-electric cells 11, 12, at the transmitter are connected to an amplifier A81, Fig. 5, and the remaining cell 10 to an amplifier A82 in order to minimize the length of the electrical connections and consequent disturbances: the cathodes rather than the anodes are earthed through the photo-electric batteries p to minimize earth-capacity effects. The amplifiers are housed in copper casings within a sheet-iron, felt and sheet-copper housing, and the first valve of each amplifier is soldered direct to the leads and suspended in an air-tight glass container which is packed with cotton-wool inside a copper cylinder and suspended by a single rubber band. swinging being checked by a dragging rubber tape. The amplifier cases are supported on soundproof pads inside the screening chamber 73. In view of the many stages of amplification, the later stages are coupled through resistances 97 and condensers 96 which give a gradual cut-off of low-frequency components as their frequencv falls below 10 cycles. At the receiving end the varying d.c. component, or average signal strength, is re-introduced manually by adjusting at 99, Fig. 4, the operating point of the valve 98 and by adjusting the input grid-bias potentiometer at 99<a>.